Use rvalue overload of Future::then in retrying

Summary:
Overall plan to modify Future<T>::then to be r-value qualified and use Future<T>::thenTry or Future<T>::thenValue.

The goal is to disambiguate folly::Future and to improve type and lifetime safety of Future and its methods.

Reviewed By: yfeldblum

Differential Revision: D9273440

fbshipit-source-id: f1e1b91fd8fe02020a5919b9193644e256413982

folly/futures/Retrying.h

@@ -84,7 +84,7 @@ void retryingImpl(size_t k, Policy&& p, FF&& ff, Prom prom) {
   using F = invoke_result_t<FF, size_t>;
   using T = typename F::value_type;
   auto f = makeFutureWith([&] { return ff(k++); });
-  f.then([k,
+  std::move(f).then([k,
                      prom = std::move(prom),
                      pm = std::forward<Policy>(p),
                      ffm = std::forward<FF>(ff)](Try<T>&& t) mutable {
@@ -94,7 +94,7 @@ void retryingImpl(size_t k, Policy&& p, FF&& ff, Prom prom) {
     }
     auto& x = t.exception();
     auto q = makeFutureWith([&] { return pm(k, x); });
-    q.then([k,
+    std::move(q).then([k,
                        prom = std::move(prom),
                        xm = std::move(x),
                        pm = std::move(pm),